This invention relates to steam turbines and more particularly to a system for sealing and venting the high pressure end thereof.
For efficiency reasons the United States utility industry requires multivalve variable inlet nozzles steam flow area control for turbine generators. As a result individual valves provide steam flow to individual inlet nozzle chambers and their associated nozzle vanes. Inlet nozzle vanes and the first rotating blade row are combined to form a control stage. Nozzle exit steam in the axial space between the nozzle vanes and the rotating blades can flow along a leakage path between nozzle seal strips and the rotor at both the base of the nozzle and the outer diameter of the blade shroud. The quantity of leakage has an effect on efficiency since no work has been done by this steam that has bypassed the rotating blade row. This leakage temperature for conventional fossile turbo generators is in the vicinity of 970.degree. F. at rated power and decreases with load. Even at half rated load, this leakage steam temperature is above 900.degree. F.
In high pressure turbines this high temperature nozzle exit leakage steam can flow between the rotor and the nozzle chamber assembly to the rotor thrust dummy balance piston. For a given geometry, the rotor material strength, with respect to maximum creep tangential stress, decreases with increasing temperature. Therefore, it is desirable to reduce this leakage temperature. One method of reducing the steam temperature the rotor is exposed to is shown in U.S. Pat. No. 3,206,166. In this patent the control stage flow direction is opposed to the direction of the following row of high pressure turbine blades and a venting and sealing system isolates the nozzle exit leakage steam from direct contact with the rotor. Single flow high pressure turbines and combined high pressure intermediate pressure turbines require a rotor thrust balancing dummy piston that has a diameter approximately equivalent to the average mean diameter of the blade path. The high pressure rotor thrust dummy piston is exposed to exit steam from the control stage rotating blade which over the load range is 45.degree. to 100.degree. cooler than nozzle exit steam. However, the opposed flow control stage has the disadvantage of being less efficient in delivering the steam to the following row of blades because of the loss associated with turning the flow 180.degree. around the nozzle chamber to the following rows of blades. It is desirable to utilize the efficiency of the straight through flow control stage and at the same time bathe the rotor in steam significantly cooler than inlet nozzle exit steam.
U.S. Pat. No. 4,150,917 shows rotor cooling for single and double axial flow steam turbines which utilizes motive steam taken from the motive steam flow path before and after the control stage or first row of rotating blades.
The invention hereinafter described through the unique arrangement of turbine internal parts and features provides the efficiency advantage of straight through flow control stage and a significantly cooler steam supply bathing the rotor in the critical area of the nozzle chamber and thrust dummy balance piston. In addition the nozzle exit leakage steam quantity at the base seal is significantly reduced further increasing control stage efficiency.